Heated fuser systems are used in electrostatic copy machines for fixing the thermoplastic image onto the copy sheets, such as paper, which are fed through the machine. The images are fixed by heat and pressure as the copy sheets pass between the heated fuser roll and the backup roll of the fuser roll couple. The heated fuser roll presses against the powdered side of the copy sheets while the backup roll provides a support for the sheets during this operation. In order to assure proper fixing of the images onto the copy sheets, it is necessary that the temperature of the fuser roll be maintained within restricted limits; and of particular importance, is the temperature profile of the fuser roll along the surface of the roll. The temperature from place to place along the surface of the roll must be kept uniform in order to assure acceptable fuse quality of the entire image and acceptable release reliability of the fused image from contact with the fuser roll as the copy sheets leave the couple.
As paper copy sheets are fed between the heated fuser roll and backup roll, they absorb heat from the fuser roll. Generally, the paper is fed through the roll couple in sidewise fashion; that is, with the length of the paper oriented parallel to the longitudinal axis of the fuser and backup rolls. In an electrostatic copy machine constructed for use with paper of uniform length, the length of the fuser roll and backup roll will be tailored to that of the paper. Thus, in operation, the fuser roll will be subjected to uniform heating and cooling conditions along its entire length. More specifically, the heat extracted from the fuser roll by the paper will be uniform along the axial length of the fuser roll. Also since the thermal characteristics of the paper are such as to effectively insulate the backup roll from the fuser roll, the temperature of the backup roll will remain essentially constant.
In machines adapted for use with paper of different shapes or sizes, as for example, paper of different lengths, the fuser roll couple will have a length sufficient to accomodate the longest paper size. With the paper fed through the fuser roll couple in sidewise fashion, the long paper will assure a uniform temperature profile along the length of the roll couple. Short paper, on the other hand, will leave one or both ends of the fuser roll exposed for direct contact with the backup roll. Where this happens, the backup roll will extract heat from the fuser roll at a rate determined by its own thermal characteristics. Normally, with conventional constructions of backup rolls, the heat so removed will not be at the same rate as that removed along the axial length of the fuser roll where paper is being fed through the couple. Thus, the temperature profile along the length of the fuser roll will not be maintained uniform. The direct contact of the fuser roll with the backup roll will also heat the end or ends of the backup roll along incremental zones as direct nip contact is made with the fuser roll. Heat will tend to collect in these zones so that as they again come into nip contact with the fuser roll, they will be hotter than the previous time and therefore not able to extract as much heat as the previous time. Thus, not only may a different quantity of heat be removed from the ends of the fuser roll, due to direct contact with the backup roll, than is extracted by the paper, the amount of heat extracted at the roll ends will vary each time each zone on the backup roll makes repeated contact with the fuser roll.
In machines constructed for use with paper of different lengths, attempts have been made to prevent the creation of any temperature differential along the axial length of the fuser roll. As an example, it has been proposed to add separate temperature control devices for maintaining the temperature of the fuser roll uniform throughout its length. Such devices are, however, generally expensive and add to the bulk of the machine. Such devices also require additional controls for activating them at the appropriate time when short paper is being fed through the machine and for deactivating them when long paper is used. Another approach which has been suggested is the use of a dual length heating element for heating the fuser roll. With a dual length heating element, the fuser roll would be heated along a length corresponding to the length of the paper being fed through the fusing couple. Again, such a device requires controls for activating it at the appropriate time. It also adds to the overall cost of the machine and presents structural problems.